The electron spin dynamics of Ⅲ-Ⅴsemiconductor InP and Ⅱ-Ⅵ semiconductor CdTe are studied by using femtosecond time resolved circularly polarized pump probe spectroscopy. Experimental results show that the spin relaxation time in CdTe with a few picoseconds is faster than that in InP. With the increase of the excitation photon energy,the spin relaxation time constant decreases monotonically. However,with increasing the carrier concentration,the spin relaxation time constant first increases,reaches a maximum and then decreases. This result coincides with theoretical prediction based on the microscopic kinetic spin Bloch equations in which Coulomb interaction is considered.